HRC#1 : Gain, Noise, Linearity, Saturation

AUTHORS : A.R. Martel, G. Hartig, and M. Sirianni

GOAL :

The main goal is to measure the primary system gains of 1, 2, 4 and
8 e-/DN with the photon-transfer method as well as the read noise for
each amplifier of the HRC flight build 1 detector. The full well of
the detector can also be estimated.

LOCATION AND DATE :

A thorough set of flat fields at gains=1, 2, and 4 were acquired on
3
Apr 2001 at GSFC/SSDIF with SMSs JGCH32A and JGCH32B for all four
amplifiers. During the Thermal
Balance/Vacuum Campaign 3, additional internal flat fields were
acquired at gains=1, 2, and 4 with the CCD basic monitor SMSs JTVH01A,
JTVH01B, and JTVH01C in Cold Soak#1 (Jul
13) and in the subsequent calibration phase, JTVH01B (gain=2) and
JTVH01D (gain=8) were executed (Jul
17). All the thermal vaccuum flat fields were acquired with amp C
only and a bias offset of 3.

Pairs of bias frames were acquired as part of SMSs JGCH32A and
JGCH32B as well as in the calibration phase of TB/TV 3 campaign on
Jul 20 (gain=1, 2) and Jul
21 (gain=4, 8) for all amplifiers.

The flat fields are acquired with the SMSs JGCH32A (gain=2) and
JGCH32B (gain=1,4). The illumination is provided by the internal
tungsten 4 lamp through the F555W filter. A brief description of the
photon-transfer method is given in the Appendix of WFC#4 : Gain, Linearity,
Saturation. A sequence of flat field pairs is acquired at each
gain setting, over as wide a range in count levels as possible, from
the read-out noise at low levels (usually at the shortest integration
time of 0.1 sec), through the linear part of the curve, and up to
non-linearities and saturation at the highest levels. Full datasets
are acquired consecutively with each amplifier. The bias level of
each image was subtracted from the virtual overscan. We find that
nearly identical results are obtained when a full median bias frame,
constructed from the bias frames acquired with the SMSs, is subtracted
from each flat field and the residuals removed with the virtual
overscan. The statistics are measured on ten 20x20 boxes distributed
over each quadrant. The gain is measured from a linear fit to the
photon-transfer curve. Since the on-orbit gain will be set to 2
e-/DN, SMS JGCH32A has the most complete coverage of the
photon-transfer curve while SMS JGCH32B only samples a few points at
gains of 1 and 4 for comparison.

The gains were also measured with the flat fields acquired in TB/TV
3 with the basic CCD monitor SMSs (amp C only) to verify those
measured from the above SSDIF flat fields and in particular, to
accurately measure gain=8. The read-out noise is derived from the
imaging area of pairs of bias frames acquired either in SSDIF or in
the dark environment of TB/TV3.

RESULTS :

The system gains and read-out noise of each amplifier for gain=1,
2, 4, and 8 e-/DN are listed in Tables 1 to 4 below. For comparison,
the measurements for both the SSDIF and TB/TV 3 environments are
tabulated. Plots of the photon-transfer curves are shown in Figs 1-3
(SSDIF) and 4-7 (TB/TV 3). Some points of interest :

Even at the shortest integration time of 0.1 sec with F555W, the
flat region of the photon-transfer curve, where the noise dominates at
low signal levels, is not reached for any gain. At gain=2, the lowest
level reached is ~110 DN (225 e-) in 0.1 sec. Lower signal levels
could be reached with a narrower filter.

The photon-transfer curves remain very linear up to the full well
i.e. there is no noticeable flattening at high flux levels. Better
sampling at these high levels may help resolve this plateau.

ADC saturation (65535 DN, log(65535)=4.82) is reached before the
full well for gain=1 and possibly for gain=2. The full well can be
reached for gain=4. From the gain=4 photon transfer curve, we estimate
the full well near 135000 e- (32500 DN). See Fig. 8.

In TB/TV 3, the noise is consistently lower than in SSDIF. This
can probably not be entirely attributed to differences in detector
temperature; in both environments, the HRC temperature was very close
(-85 C to -84.3 C in SSDIF and -84.3 C to -80.4 C in TB/TV 3). One
possibility is that the imaging area of the bias frames acquired in
SSDIF suffers from straylight contamination from the ceiling lights
during read-out, although we found that the ACS enclosure is
light-tight (see Enclosure Light Leak
Characterization). The TB/TV 3 noise measurements should serve as
the primary reference.

Table 1 : Gain in e-/DN (SSDIF)

Amplifier

Gain="1"

Gain="2"

Gain="4"

A

1.06 +/- 0.01

2.05 +/- 0.01

4.05 +/- 0.04

B

1.09 +/- 0.01

2.12 +/- 0.01

4.20 +/- 0.03

C

1.18 +/- 0.01

2.22 +/- 0.01

4.29 +/- 0.04

D

1.11 +/- 0.01

2.09 +/- 0.01

4.14 +/- 0.04

Average

1.105 +/- 0.005

2.118 +/- 0.007

4.17 +/- 0.02

Table 2 : Gain in e-/DN (TB/TV 3)

Amplifier

Gain="1"

Gain="2"

Gain="4"

Gain="8"

Jul 13

Jul 17

C

1.15 +/- 0.01

2.26 +/- 0.03

2.25 +/- 0.04

4.36 +/- 0.07

8.68 +/- 0.17

Table 3 : Read-Out Noise in e- (SSDIF)

Amplifier

Gain="1"

Gain="2"

Gain="4"

A

5.33 +/- 0.26

4.93 +/- 0.30

6.18 +/- 0.28

B

4.54 +/- 0.22

5.00 +/- 0.66

5.85 +/- 0.18

C

4.52 +/- 0.18

4.68 +/- 0.17

5.71 +/- 0.25

D

4.87 +/- 0.24

4.69 +/- 0.21

6.17 +/- 0.28

Average

4.75 +/- 0.11

4.73 +/- 0.12

5.93 +/- 0.12

Note : The read noise is measured in DN and is converted to e-
using the gains measured in SSDIF.

Table 4 : Read-Out Noise in e- (TB/TV 3)

Amplifier

Gain="1"

Gain="2"

Gain="4"

Gain="8"

A

4.41 +/- 0.16

4.60 +/- 0.19

5.65 +/- 0.25

-

B

4.17 +/- 0.13

4.40 +/- 0.13

4.40 +/- 0.11

-

C

4.51 +/- 0.16

4.74 +/- 0.18

4.60 +/- 0.07

12.01 +/- 0.57

D

4.70 +/- 0.15

4.98 +/- 0.19

5.78 +/- 0.18

-

Average

4.42 +/- 0.07

4.62 +/- 0.08

4.71 +/- 0.06

-

Note : The read noise is measured in DN and is converted to e-
using the gains measured in SSDIF (gains=1, 2, and 4) and in TB/TV
3 (gain=8 for amp C only).

Fig. 8 : ID 28280
(Gain=4, 70 sec, F555W, amp C, full frame) : This exposure level has
reached saturation and the full well of the CCD, as evidenced by the
vertical streaking which dominates in regions furthest from the
read-out amplifier (amp C in the top-left corner).

CEI SPECIFICATIONS :

In STE-50, "Information End Item Specification (Part II)", Table
4-6 states that the requirement for the RMS noise per pixel, which
includes both the read noise and noise from the dark current, is <4.5
e- and the goal is <3.5 e- for a reference integration time of 13 min
(780 sec). The read noise was measured from the imaging area of the
bias frames and is listed in Tables 3 and 4 above for different
amplifier and gain combinations. The dark count rate measured in
TB/TV 3 is approximately 8 e-/pix/hour, which amounts to ~1.7 e-/pix
for a 780 sec exposure. The noise on the dark rate is taken as shot
noise i.e. the square root of the dark current (as in the ACS Exposure
Time Calculator, see p. 103 of the ACS user manual), so the total
noise is the square root of the sum of the read noise squared and the
dark rate (1.7 e-/pix). From Table 4, we find that only amp B at
gain=1 meets the CEI specification (total noise of 4.37 e-). None of
the other amplifier/gain combinations meet the noise requirements,
including the default on-orbit configuration, amp C at gain=2. In the
same CEI table, the full well is specifed as >99900 e-. Our measured
full well of ~135000 e- meets this requirement.

CONCLUSION :

The system gain (e-/DN) and total noise (e-) of the primary gain
settings of 1, 2, 4, and 8 for the HRC build 1 detector were
measured. The full well is estimated at ~135000 e-. The gains from
SSDIF (Table 1) and the noise measurements from TB/TV 3 (Table 4) are
the primary reference.